hello there... I feel the same as you, the japanese grammar and the pronunciation is a piece of cake, but the kanji y and the respective reading (kun/on) is my pitfall. but in the other hand the English pronuniation can be the most difficult thing for me (and I think for the majority (or most?) of native Spanish) when I try to make an approach to this lenguage.
...by the way, the expretion ''piece of cake'' in spanish is: pan comido.